Plant cells can be grown in isolation from intact plants in tissue culture systems. The cellshave the characteristics of callus cells, rather than other plant cell types. These are thecells that appear on cut surfaces when a plant is wounded and which gradually cover andseal the damaged area.Pieces of plant tissue will slowly divide and grow into a colourless mass of cells if theyare kept in special conditions. These are:

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initiated from the most appropriate plant tissue for the particular plant variety

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presence of a high concentration of auxin and cytokinin growth regulators in thegrowth media

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a growth medium containing organic and inorganic compounds to sustain the cells

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aseptic conditions during culture to exclude competition from microorganismsThe plant cells can grow on a solid surface as friable, pale-brown lumps (called callus),or as individual or small clusters of cells in a liquid medium called a suspension culture.These cells can be maintained indefinitely provided they are sub-cultured regularly intofresh growth medium.Tissue culture cells generally lack the distinctive features of most plant cells. They have asmall vacuole, lack chloroplasts and photosynthetic pathways and the structural or chemical features that distinguish so many cell types within the intact plant are absent.They are most similar to the undifferentiated cells found in meristematic regions which become fated to develop into each cell type as the plant grows. Tissue cultured cells canalso be induced to re-differentiate into whole plants by alterations to the growth media.Plant tissue cultures can be initiated from almost any part of a plant. The physiologicalstate of the plant does have an influence on its response to attempts to initiate tissueculture. The parent plant must be healthy and free from obvious signs of disease or decay.The source, termed explant, may be dictated by the reason for carrying out the tissueculture. Younger tissue contains a higher proportion of actively dividing cells and is moreresponsive to a callus initiation programme. The plants themselves must be activelygrowing, and not about to enter a period of dormancy.

The exact conditions required to initiate and sustain plant cells in culture, or to regenerateintact plants from cultured cells, are different for each plant species. Each variety of aspecies will often have a particular set of cultural requirements. Despite all theknowledge that has been obtained about plant tissue culture during the twentieth century,these conditions have to be identified for each variety through experimentation.

PLANT TISSUE CULTURE

Uses of plant tissue culture

Callus cultures derived from onion

Plant tissue culture now has direct commercial applications as well as value in basicresearch into cell biology, genetics and biochemistry. The techniques include culture of cells, anthers, ovules and embryos on experimental to industrial scales, protoplastisolation and fusion, cell selection and meristem and bud culture. Applications include:

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micropropagation using meristem and shoot culture to produce large numbers of identical individuals

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screening programmes of cells, rather than plants for advantageous characters

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large-scale growth of plant cells in liquid culture as a source of secondary products

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crossing distantly related species by protoplast fusion and regeneration of thenovel hybrid

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production of dihaploid plants from haploid cultures to achieve homozygous linesmore rapidly in breeding programmes

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as a tissue for transformation, followed by either short-term testing of geneticconstructs or regeneration of transgenic plants